Mechanical properties, crack evolution and damage characteristics of prefabricated fractured cemented paste backfill under uniaxial compression

The existence of joints and fractures will inevitably affect the mechanical behavior and crack propagation law of the backfill, and then degrade the stability and accelerate the damage evolution of the backfill. Thus, in this study, the uniaxial compressive strength (UCS) tests and acoustic emission (AE) monitoring were carried out for prefabricated fractured (PF) cemented paste backfill (CPB or FCPB) with different dip angles (0, 30, 45, 60, 90 degrees) respectively. The results showed that: 1) The existence of PF collapsed the mechanical properties of the backfill. The UCS and elastic modulus of FCPB samples first decreased and then increased with the increase of PF dip angle, and the deterioration of the 30 degrees FCPB sample was the most serious scenario. 2) Apart from the stress-strain curve of the traditional CPB sample, FCPB sample (except 90 degrees) was a bimodal curve. The self-strengthening characteristics caused by the compression closure of PF led to the continuous upward trend of stress after reaching the first peak and then the second peak. 3) The crack of the FCPB sample usually originated at the tip of PF, and the far-field crack was easy to expand to PF. The PF was conducive to crack development propagation, resulting in macro failure. The failure modes of CPB and FCPB samples were shear failures and tensile failure respectively (shear crack and tensile crack were significantly affected by PF dip angle). 4) The AE ringing count rate and energy rate of FCPB samples (except 90 degrees) usually surged twice, which corresponded to the vicinity of the two peak points of the stress-strain curve of FCPB samples. 5) A piecewise statistical damage constitutive model considering initial damage was established, which could fully reflect the bimodal stress-strain curve characteristics of FCPB samples. The damage evolution law of the FCPB samples obtained from this was significantly affected by PF dip angle. The damage curves of CPB and FCPB (60, 90 degrees) increased approximately in S-shape, while the damage curves of other FCPB increased continuously in the first peak stage and then entered the stable development- accelerated damage-failure stage of the second peak stage. The findings of the study can serve as a theoretical foundation for analyzing the stability of backfill with geological defects such as joints and fractures.